Navigation using global positioning satellites (GPSs) has become increasingly commonplace in today's society. All types of devices from cellular telephones to automobiles to exercise watches have begun to implement GPS location and tracking systems. These systems allow users of these devices to know where they are in relation to the earth, and where they heading. Using triangulation between different satellites, the devices can determine the user's current position, velocity, direction and even elevation. Such devices are, thus, very useful in areas where satellite reception is good.
However, in many areas, such as urban areas with tall buildings, tunnels surrounded by concrete, or in mountain valleys, GPS units do not work due to limited or no reception. Because the satellites cannot communicate with the GPS-enabled device, the device will display an indication to the user that GPS functionality is no longer working. At this point, the GPS device does not know where the user is or where the user is going.
In such GPS-denied environments, stand-alone inertial motion units (IMUs) have traditionally been used to determine which way the user is moving. IMUs typically combine an array of gyros and an accelerometer to detect the user's movements and extrapolate an estimated path of travel. However, over a short amount of time, these IMUs lose accuracy and wrongly determine the user's path of travel.